1. Field of the Invention
The present invention relates to an apparatus for and a method of forming an electrophotographic image, and more particularly to those adapted for various business machines and instruments, especially for a storage device such as a printer.
2. Description of the Prior Art
An electrophotographic image processing apparatus has been conventionally utilized as an electrophotographic printer. Such an image processing system carries out the steps of charging an image carrier, i.e. a photoconductor drum with electricity uniformly, forming a latent image on the photoconductor drum, developing the latent image by toner, transferring the toner on the photoconductor drum to a transfer member, fixing the toner on the transfer member and removing the toner remaining on the photoconductor drum therefrom. There is a technique to remove electricity before the next charging process starts upon completion of the transferring process in order to prevent an afterimage from being formed on the photoconductor drum. The charging process and the transferring process are generally performed by utilizing corona discharge.
Since a harmful substance such as ozone produces through the corona discharge, it has been collected by a filter, etc. However, the use of the filter for a long time causes the degraded collective efficiency and the frequent filter replacement.
An ozone free process where ozone is prevented from generating by employment of a roller type transfer system or a charging roller system is proposed. (refer to Electronic Communication Institute Thesis '77/4 Vol. J60-C NO. 4 pp 213-218).
The roller type transfer system performs the steps of placing a transfer member on a toner image formed by development on the surface of a photoconductor drum, pressing a transfer roller on the transfer member and applying a voltage polarity of which is opposite to that of the toner, to the transfer roller. In this system, an electric field is generated in a gap between the transfer member and the upper layer of the toner image whereby the toner is transferred to the toner member by an electrostatic force of the electric field.
The charging roller system has the same principle as the roller type transfer system for charging the photoconductor drum with electricity. In this system, a voltage is applied to a charging roller so that an electric charge is directly applied to the photoconductor drum, which leads to no generation of ozone.
There is proposed an image forming system eliminating a cleaning process (Refer to Japan Hardcopy '89 Thesis pp 143-146). In this system, the photoconductor drum is exposed to light after the photoconductor drum is uniformly charged with electricity by the corona discharge whereby the surface potential of the exposure portion is attenuated. Toner is stuck to the attenuated portion by reversal development while toner in a thin layer which remains on the photoconductor drum is collected therefrom. That is, since the toner which remains on the nonexposure portion of the photoconductor drum after the completion of the transferring process is charged with electricity with the same polarity as that in the developing process, the toner is attracted by the developing unit owing to the electrostatic force caused by the difference between the surface potential of the photoconductor drum which is charged with electricity and the developing bias.
The employment of this cleaningless process can miniaturize the image processing apparatus and can recollect the remaining toner in the developing process. Therefore the toner is not necessary to be disposed of and can be reused with high efficiency.
However, since the ozone free process collects the remaining toner by a cleaning blade or a brush or the like, the collected toner should be disposed of. Furthermore, in the cleaningless process, since the efficiency of the filter for collecting harmful substance is gradually lowered as the time lapses, the filter has to be maintained and controlled by proper replacement, which becomes troublesome.
Accordingly, even if both the processes are combined with each other, since the photoconductor drum contacts the charging roller while the toner remains stuck to the photoconductor drum after the transferring, the toner is attracted to the charging roller with ease, whereby the remaining toner is difficult to be collected in the developing process, thus leading to deterioration of the printing quality.